A similar type of hydraulic damper, has been developed to provide a system as shown in FIG. 6, for example.
This type of hydraulic damper is constructed such that a piston rod 3 is movably inserted into a cylinder 1 through a piston 2 acting as a partition wall member. The piston 2 defines two hydraulic chambers, i.e. an upper hydraulic chamber 4 and a lower hydraulic chamber 5 in the cylinder 1. The two hydraulic cylinders 4 and 5 communicate with each other through an extension port 6 and a compression port 7 arranged in the piston 2.
At an outlet side of the extension port 6 there are formed two seats (a) and (b) having different heights to each other. The seat (a) is provided with a leaf valve 8 for a fine slow speed supported by an intermediate seat 17 in such a way as it may be opened or closed. The other seat (b) is provided with a cutout leaf valve 9 in such a way as it may be opened or closed. Below the cutout leaf valve 9 there is arranged a sub-leaf valve 10 supported by a valve stopper biased by a spring 14.
At the upper outlet seat (c) of the compression port 7, a cutout leaf valve 12 is formed with a recess 15 and a communication hole 16, and a leaf valve 11 in such a way as they may be opened or closed.
In case of an extending operation, hydraulic oil in the upper hydraulic chamber 4 flows into the lower hydraulic chamber 5. At the fine slow speed range, the leaf valve 8 is opened to cause the hydraulic oil to flow out through the cutout at an outer circumference of the cutout valve 9. In turn, at the intermediate and high speed ranges showing an increased flow rate, the leaf valve 8 is flexed downwardly and at the same time the cutout leaf valve 9 is flexed by an amount where set loads of the sub-leaf valve 10 and a spring 14 are balanced with the flow rates so as to generate an attenuation force. However, in case of the aforesaid hydraulic damper, as the set load of the sub-leaf valve 10 under an opened valve state, a plate thickness of the sub-leaf valve 10 itself and a step difference between the seats (a) and (b) are accumulated in addition to the set load of the spring 14, an initial load of the sub-leaf valve 10 is high and a disturbance of attenuation force caused by the plate thickness and a tolerance of the step difference is increased.
Also in the example shown in FIG. 6, the fluid will bend the leaf valve 8 until the leaf valve 8 contacts the leaf valve 9. When leaf valve 9 bends, leaf valve 8 can then bend farther. This further effects and complicates the calculation of the attenuation force.
In addition, a repetitive flexing of the leaf valve 8 may cause stress to be concentrated at a location near the intermediate seat 17 and a crack caused by a buckling action may be generated.
In U.S. Pat. No. 4,964,493 to Yamura et al., describes an upper disc valve 138 with a stopper plate 144. A lower disc valve 140 abuts, and is biased against a complete side of this stopper plate 144. Fluid flowing past the upper disc valve 138 only initially can come into contact with a small outer periphery of the lower disc valve 144. It may be possible that after the lower disc valve 140 begins to open the fluid can come into larger contact with the lower disc valve 140. This changing of the fluid contact area changes the amount of force that the lower disc valve 140 receives from the fluid, therefore changing the amount that the lower disc valve 140 opens and the attenuation force. The opening of the lower disc valve 140 is therefore not smooth and correspondingly so is not the attenuation. The attenuation force being very difficult to calculated because of all these changes.
In the example shown in FIG. 6, the fluid flowing past leaf valve 8 also only acts on an outer periphery of leaf valve 9, especially when the leaf valve 8 is fully open. The amount of area of the leaf valve 9 that is in contact with the fluid can vary depending on the position of the leaf valve 8. Again this causes changes in the amount of opening of the leaf valve 9 and changes in the attenuation force. A smooth attenuation force is then prevented by this change in fluid contact area.